To prevent a watervehicle from approaching and colliding with a structure, such as a offshore drilling rig or an artificial island, constructed at sea, conventionally, display buoys or the like are spread on the surface of water in a warning water area set around the structure. Such a display buoy alone cannot continuously define and show a warning water area. In case of overlooking such a display buoy, therefore, a watervehicle is likely to enter the warning water area and collide with or contact a marine structure.
As a solution to the problem, a watervehicle intrusion preventing fence which defines a warning water area around a marine structure in a line has been proposed in Unexamined Japanese Utility Model Application KOKAI Publication No. H2-115791. This fence includes elongated beltlike floating bodies disposed at a sea surface concentrically around a marine structure, a net laid out between the beltlike floating bodies, and anchor which fixes the beltlike floating bodies and the net.
According to the watervehicle intrusion preventing fence, the bow of a watervehicle hits the beltlike floating bodies to be caught, or the propeller of the watervehicle is caught with the laid-out net, so that the anchor is dragged to decelerate or stop the watervehicle.
Water pollution (turbid) which is caused by stirring a sea soil picked up in a dredging work or reclamation work which is carried out at sea or the like may enter gills of fishes to cause respiratory distress and reduce the illuminance in water to adversely affect benthic creatures. To overcome those problems, a water pollution preventing membrane which includes a floating body membrane structure is used as a structure which is laid out to prevent diffusion of water pollution. The layout of this water pollution preventing membrane can physically prevent diffusion of water pollution (turbid) which occurs in a dredging work or reclamation work carried out at sea or river.
The watervehicle intrusion preventing fence disclosed in Unexamined Japanese Utility Model Application KOKAI Publication No. H2-115791 is premised on navigation of a large ship and is intended to prevent intrusion of such a ship with the fence. A meshed net is planarly laid out near a sea surface. Depending on the mesh size of the net, therefore, when a ship passes over the net, the bottom of the ship may push down the entire net. As a result, the propeller of the ship may pass over the net, or the eddy produced around the propeller may push away the net so that the ship enters the warning water area.
Large ships are not navigated in a sea area where the water pollution preventing membrane is located. Therefore, the shock resistance of the floating body of the floating body membrane structure prevents damage or the like from being caused by collision of a small watervehicle. To improve the safety of a floating body membrane structure, however, it is necessary to prevent collision of a watervehicle with the floating body beforehand.